1) Field of the Invention
The present invention relates to plasma arc torches and, in particular, to plasma arc torches wherein an electrode and an electrode holder are held to each other or to the torch by way of a threaded connection.
2) Description of Related Art
Plasma arc torches are commonly used for the working of metal including cutting, welding, surface treatment, melting and annealing. Such torches include an electrode that supports an arc that extends from the electrode to a workpiece in a transferred-arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc in a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein that supports the arc. The opposite end of the electrode holds the electrode in the torch by way of a threaded connection to an electrode holder. The electrode holder is typically an elongate structure held to the torch body by a threaded connection at an end opposite the end at which the electrode is held. The electrode holder and the electrode define a threaded connection for holding the electrode to the electrode holder.
The emissive element of the electrode is composed of a material that has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (eV), which promotes thermionic emission from the surface of a metal at a given temperature. In view of this low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and alloys thereof.
A nozzle surrounds the discharge end of the electrode and provides a pathway for directing the arc towards the workpiece. To ensure that the arc is emitted through the nozzle and not from the nozzle surface during regular, transferred-arc operation, the electrode and the nozzle are maintained at different electrical potential relative to each other. Thus, it is important that the nozzle and the electrode are electrically separated, and this is typically achieved by maintaining a predetermined physical gap between the components. The volume defining the gap is most typically filled with flowing air or some other gas used in the torch operation.
The heat generated by the plasma arc is great. The torch component that is subjected to the most intense heating is the electrode. To improve the service life of a plasma arc torch, it is generally desirable to maintain the various components of the torch at the lowest possible temperature notwithstanding this heat generation. A passageway or bore is formed through the electrode holder and the electrode, and a coolant such as water is circulated through the passageway to cool the electrode.
Even with the water-cooling, the electrode has a limited life span and is considered a consumable part. Thus, in the normal course of operation, a torch operator must periodically replace a consumed electrode by first removing the nozzle and then unthreading the electrode from the electrode holder. A new electrode is then screwed onto the electrode holder and the nozzle is reinstalled so that the plasma arc torch can resume operation.
The design of the threaded connection between the electrode holder and the electrode must take into account various constraints. First, the threaded connection must be structurally strong enough to securely hold the electrode to the electrode holder. Second, in the case of water-cooled torches, the threaded connection should allow for sealing between the electrode holder and the electrode so that the cooling water cannot escape. The sealing is typically achieved by way of an o-ring, and so the threaded connection should allow sufficient room for such an o-ring. Third, a considerable current is passed through the electrode holder to the electrode, in some cases up to 1,000 amperes of cutting current. Thus, the threaded connection should provide sufficient contact surface area between the electrode and the electrode holder to allow this current to pass through. Finally, the cost of manufacturing the electrode should be as small as possible, especially because the electrode is a consumable part. Similar considerations exist with respect to the threaded connection holding the electrode holder to the torch body.
One way that this cost can be reduced is to make the electrode shorter, thus reducing material cost and manufacturing cost. This can be achieved by making the electrode holder longer to compensate for the shorter length of the electrode so that the total length of the electrode holder and electrode remains the same. However, the length of the electrode holder is limited by the nozzle geometry because the threaded connection between the electrode holder and the electrode in many conventional torches is too large to extend into the nozzle chamber and still meet the design constraints noted above.
In particular, the threaded connection in present designs sometimes comprises an enlarged female-threaded portion at the end of the electrode holder that is radially larger than the adjacent male-threaded end of the electrode. Thus, if such a conventional threaded connection were designed to extend into the nozzle, then the gap between the electrode holder and the nozzle would decrease. As noted above, the electrode and electrode holder are at one electrical potential and the nozzle is at a different electrical potential. Thus, the decrease in the gap might cause undesired arcing within the torch from the nozzle to the electrode holder.
This particular problem has been resolved in part in some prior torches by forming a threaded connection using a male thread for the electrode holder and a female thread for the electrode. One advantage of this approach is that the electrode holder is protected from damage because any arcing that does occur inside the torch extends from the outside of the electrode to the nozzle, and not from the electrode holder to the nozzle, because the outer surface of the female-threaded portion of the electrode is radially closest to the remainder of the torch. Because the electrode must be periodically replaced when the emissive end is spent in any event, damage to the threaded end of the electrode is less of a concern than it is to the electrode holder.
One disadvantage of this approach, however, is that female threads are generally more difficult to machine and thus are more expensive than male threads. Even though the electrode holder can sometimes be a consumable part, the rate of consumption is typically less than that of the electrode, and thus this configuration can have an undesirable cost structure. The more frequently replaced part must be subjected to the more expensive of the two machining operations necessary for making a threaded connection.
Another way to resolve at least some of these design constraints is to use a fine thread. A fine thread allows a shorter thread height (i.e. the dimension of the thread in the radial direction) than a corresponding coarser thread as used in conventional torches. This reduced thread height allows more of a gap between the threaded connection and the nozzle. However, fine threads are more difficult to machine and thus can be more expensive. In addition, fine threads are more delicate, are quicker to become unusably worn on the electrode holder when electrodes are repeatedly replaced, and are more likely to be improperly cross-threaded when an operator is installing a new electrode.
Thus, there is a need in the industry for an electrode and an electrode holder where the threaded connection therebetween is capable of meeting all of the electrical, structural and sealing constraints required in a plasma arc torch, but yet which is capable of being positioned at least partially within a nozzle of the plasma arc torch without detrimental arcing occurring between the threaded connection and the nozzle. Such a threaded connection would preferably be relatively easy to manufacture and would involve limited risks of cross-threading when the electrode is attached to the electrode holder.
In addition, it would be desirable to provide an electrode that can be secured to the electrode holder by way of a threaded connection where the machining and material costs, and the possibilities of premature wear and damage, are reduced for the electrode. Because the costs and possibility for damage in such an arrangement would be distributed more to the more-consumable electrode than to the less-consumable electrode holder, the long-term costs of operating the plasma arc torch would be reduced. Similar advantages would also be beneficial for the threaded connection between the electrode holder and the torch body.